1. Field of the Invention
The present invention relates generally to acoustoelectric signal processing devices and, more particularly, to a new and improved monolithic acoustoelectric device having a holographic signal waveform storage capability. The device is useful for storage alone and/or for such signal processing applications as convolution, correlation and the like of relatively high frequency signals.
2. Description of the Prior Art
Considerable effort has been expended in recent years to utilize the unique properties of acoustic wave devices in the signal processing sections of apparatus for radar, communication systems and the like. This is due to their properties of relatively small size, simple fabrication techniques, high reliability and satisfactory electrical performance when compared with alternate devices. Some surface acoustic wave devices, and applications therefor, are reviewed generally in an article by Gordon S. Kino and John Shaw in the October 1972 issue of Scientific American, pages 51-68, entitled "Acoustic Surface Waves". This Kino and Shaw article is hereby incorporated by reference into this specification for its discussion of surface acoustic wave phenomena generally, for its discussion of various prior art surface acoustic wave device configurations, and for its discussion of some signal processing applications of such devices.
Among the signal-processing functions which may be accomplished using surface acoustic wave devices, as discussed by Kino and Shaw, are the detection of biphase coded radio frequency (rf) signal pulses, pulse compression of chirped rf signals, signal storage by extended delay, convolution, and correlation including autocorrelation and cross-correlation.
The usefulness of acoustoelectric devices for accomplishing the above-mentioned signal processing functions and others for various applications can be extended by an acoustic wave device having memory, i.e., a capability incorporated therein for storing for a substantial period of time a replica of a signal waveform. In such a device, after such storage is accomplished, the stored signal may then be read out or an output signal may be generated which is the result of the interaction of a spatial distribution of trapped charge conforming to the stored waveform with the electric field associated with a subsequently generated surface acoustic wave.
A surface acoustic wave device having storage capabilities, as discussed above, and various ways of operating it are described in the following three recently published papers, each one of which is hereby incorporated by reference into this specification: (1) J. H. Cafarella, A. Bers and E. Stern, "Surface Acoustoelectric Correlator with Surface State Memory", 1974 Ultrasonics Symposium Proceedings, IEEE Cat. No. 74 CHO 896-1SU, pages 216-219; (2) Abraham Bers and J. H. Cafarella, "Surface Wave Correlator-Convolver with Memory", 1974 Ultrasonics Symposium Proceedings, IEEE Cat. No. 74 CHO 896-1SU, pages 778-787; and (3) H. Hayakawa and D. S. Kino, "Storage of Acoustic Signal in Surface States in Silicon", Applied Physics Letters, Vol. 25, No. 4, Aug. 15, 1974, pages 178-180.
The device described in each of the three publications referenced immediately above comprises a substrate of piezoelectric material capable of supporting and propagating surface acoustic waves. Standard interdigital transducers capable of exciting (and receiving) surface acoustic waves are disposed on the upper or inner surface of the substrate at the opposite ends of a longitudinal surface wave propagation path thereon. A semiconductor wafer of silicon is positioned adjacent the propagation path on the inner surface of the substrate intermediate the interdigital transducers. The semiconductor wafer is spaced apart from the substrate to avoid interfering with surface waves thereon. This spacing introduces an air gap between the lower or inner surface of the water and the inner surface of the substrate. A first metal electrode is disposed on the lower or outer surface of the substrate. A second metal electrode is disposed on the upper or outer surface of the silicon wafer. The first and second electrodes provide means for connection to external circuitry for effecting the storage or read-out of signals. In this device, a holographic replica of the waveform of a signal transduced into a surface acoustic wave is stored as a spatially varying distribution of charge trapped in surface states of the semiconductor wafer.
While some success in signal processing has been achieved with the above-described acoustoelectric storage device, a configuration which requires a semiconductor wafer to be precisely spaced across an air gap from a piezoelectric substrate is difficult and expensive to manufacture in quantity to the tolerances required for consistent results. In addition, the retention times for storing signals by semiconductor surface state free-carrier trapping is inherently relatively short.